Barrier membranes at the outer surface of the brain of an elasmobranch, Raja erinacea

This report gives the results of the first electron-microscopic examination of the cell layers covering the outer brain surface and the inner surface of the cartilaginous skull in the skate, Raja erinacea. The perivascular glial blood-brain barrier--a characteristic of elasmobranchs--extends to the outer surface of the brain. This outer barrier layer is surrounded, in turn, by a subarachnoid compartment (depth: 30-40 microns), containing loose connective tissue and blood vessels; by an arachnoid-like epithelium (10-15 cell layers), impermeable to horseradish peroxidase; and, by perimeningeal fluid, a fluid with a slow turnover rate and a protein composition different from plasma. The inside of the skull, facing the perimeningeal fluid, is covered by a multilayered (10-15 layers) cuboidal epithelium, also impermeable to horseradish peroxidase. Closely apposed cells in the luminal layer of this epithelium have apical microvilli and numerous vesicular profiles, containing material of moderate electron density. These observations may explain, in terms of structure, the regulated protein content of perimeningeal fluid and the restricted exchange of solutes between brain and perimeningeal fluid in elasmobranchs.